In recent years, for example, in automobile valve springs, there is a trend of reducing dimensions and weight because it is desired to decrease friction loss of valve systems and obtain a crumple space in an engine compartment. Therefore, the strength required for a spring steel material has been increasing. In general, fatigue strength of a steel increases with hardness until the material hardness is not more than approximately 400 HV. The increasing rate of the fatigue strength slows in a high hardness range in which the hardness is greater than approximately 400 HV. Then, when the hardness is further increased, the increasing rate of the fatigue strength tends to be saturated. One reason for this is increase of notch sensitivity of the steel with the increase of the hardness. Since the hardnesses of spring steels and springs in recent years exceed 500 HV, it is expected that the fatigue strength cannot be greatly increased by increasing the hardness of the entirety of a steel material. In view of this, the following methods are disclosed in order to solve such a problem.
A spring with superior fatigue characteristics is disclosed in Japanese Unexamined Patent Application Laid-open No. 8-170152. In this case, crystal grains are refined by adding V, Nb, or the like, whereby toughness of a steel material is improved. Naturally, in this spring, while the fatigue characteristics are improved, the cost of the steel material is high because the above elements are expensive.
A technique for providing high fatigue strength is disclosed in Japanese Unexamined Patent Application Laid-open No. 8-134545. In this technique, a quenching and tempering treatment is performed by using a high-frequency induction heating. As a result, structure of a steel material is refined, and fine carbides are dispersed and precipitated so as not to precipitate at grain boundaries, whereby toughness of the steel material is improved. Since springs generally have three-dimensional shapes, it is not easy to control temperature of each portion of a steel material by using the high-frequency induction heating. Accordingly, control of the heating conditions is complicated, which results in high production cost.
A spring steel with superior fatigue characteristics is disclosed in Japanese Unexamined Patent Application Laid-open No. 6458226. The spring steel contains oxide inclusions that consist of, by weight %, 30 to 60% of SiO2, 10 to 30% of Al2O3, 10 to 30% of CaO, and 3 to 15% of MgO, and that have grain sizes of not more than 15 μm in circle-equivalent diameter. However, it is difficult to control the compositions and the grain sizes of the oxide inclusions to be in the above range. In this regard, it is necessary to inspect whether the amounts of the oxide inclusions in produced spring steels are in the above range. In spring steels other than spring steels which are inspected, even if they have the same lot, the amounts of the oxide inclusions may be out of the above range. In this case, a spring made of the spring steel has a potential of break at an early time due to the oxide inclusions.
A carbonitrided hardened material and a production method therefor are disclosed in Japanese Unexamined Patent Application Laid-open No. 2007-46088. The carbonitrided hardened material does not have brittle nitrogen compounds at a surface layer, but has a nitrogen diffused layer from the surface to a predetermined depth where nitrogen is solid solved. In addition, the carbonitrided hardened material is subjected to a quenching treatment. According to this technique, the nitrogen compounds that can become starting points of breaks are not formed after a carbonitriding treatment, and the surface layer has high hardness, whereby the fatigue strength may be improved. However, in an example disclosed in Japanese Unexamined Patent Application Laid-open No. 2007-46088, a high hardness layer at the surface layer had a thickness of approximately 0.06 mm at most and was too thin to greatly improve the fatigue strength. In this case, the carbonitriding treatment is performed at a temperature of 600 to 800° C. In this temperature range, the center portion of a steel of the present invention is difficult to be austenitized and is incompletely hardened even by a subsequent rapid cooling. Therefore, the steel cannot have a center portion with hardness of not less than 500 HV, which is necessary when used as a spring.
A carburized gear part is disclosed in Japanese Patent No. 4229609. The carburized gear part is made by carburizing under reduced-pressure atmosphere of 1 to 30 hPa and thereby has a carburized layer with surface hardness of 700 to 900 HV. The reduced-pressure carburizing is different from gas carburizing which is conventionally widely used. In the reduced-pressure carburizing, grain-boundary oxidation that can generate starting points of breaks is prevented, and a thick high hardness layer is obtained by means of high treatment temperature (950° C. or higher). In this case, since the reduced-pressure carburizing is performed by non-equilibrium decomposition reaction of acetylene, the absorbed amount of carbon cannot be controlled as well as the gas carburizing. Therefore, the absorbed amount of carbon varies depending on portions and a shape of a member that was subjected to the reduced-pressure carburizing, whereby a high hardness layer is difficult to obtain uniformly. Accordingly, a treatment method, in which the absorbed amount of carbon or the like is easily controlled, is required.